1. Field of the Invention
The present disclosure relates to a piezoelectric transducer driver configured to generate a voltage by driving a piezoelectric transducer, a power supply device including the piezoelectric transducer driver, and an image formation apparatus including the power supply device.
2. Description of Related Art
A piezoelectric transducer is a voltage transducer configured to convert an inputted alternating-current voltage and to output the converted voltage. Such piezoelectric transducers are widely used in power supply devices which are designed to generate drive voltages to be supplied to cold-cathode tubes of liquid crystal displays and the like, or to generate voltages to be supplied to transfer rollers and development rollers in electrographic image formation apparatuses, for example. An output characteristic (resonance characteristic) of a piezoelectric transducer varies depending on a change in impedance of a load such as a cold-cathode tube or a transfer roller to which the output voltage is to be supplied. Accordingly, in order to stabilize the output voltage, a frequency of the alternating-current voltage (a drive frequency) to be inputted into the piezoelectric transducer needs to be controlled in accordance with the change in the impedance of the load and the like. The drive frequency can be controlled by using an analog circuit such as a voltage-controlled oscillator (VCO). A power supply device using a VCO is disclosed in Japanese Patent Application Publication No. 2007-189880 (Document 1), for example.
However, the power supply device disclosed in Document 1 controls drive frequency in an analog mode, and therefore has problems of requiring a large number of components for the analog circuit, and having a difficulty in achieving flexible control. In this regard, drive frequency control using a digital circuit has been proposed in recent years. For example, Japanese Patent Application Publication No. 2010-148321 (Document 2) discloses: a power supply device configured to perform drive frequency control for a piezoelectric transducer in a digital mode; and an image formation apparatus including the power supply device.
The power supply device disclosed in Document 2 uses a switching element such as a power metal-oxide semiconductor field-effect transistor (power MOSFET) in order to generate an alternating-current voltage to be supplied to the piezoelectric transducer. The power supply device disclosed in Document 2 controls the drive frequency for the piezoelectric transducer within a predetermined frequency range by controlling the frequency of a drive pulse to be applied to a control terminal (a gate) of the power MOSFET. Moreover, this power supply device controls switching operation (on-and-off operation) of the power MOSFET while setting a substantially constant on-duty ratio (a ratio of a high-level period in one cycle) of the drive pulse which is to be applied to the gate of the power MOSFET and be used for driving the piezoelectric transducer. The drive frequency for the piezoelectric transducer is controlled within a range equal to or below about 130 kHz.